1. Field of the Invention
The present invention relates to a fluid cross-flow fluid-solid contactor of the panel or radial reactor type wherein ferromagnetic bed solids are structured or stabilized by the action of a magnetic field, and is particularly concerned with means for contacting fluids and solid particles in one or more such cross-flow beds having an imposed magnetic field. The invention also relates to a method for removing particulates from a gas containing the same and flue gas desulfurization processes utilizing a cross-flow or panel bed contactor stabilized by the action of a magnetic field.
2. Description of the Prior Art
There is considerable interest in the field of fluid-solids contacting, particularly gaseous fluids. Such processes have found uses in coal gasification, catalytic reactions, gas absorption, gas adsorption, and filtering particulate material from gases and flue gas desulfurization processes. Many such processes are carried out in fluidized beds, i.e., beds containing a mass of solid fluidizable particles in which the individual particles are effectively buoyed by fluid drag forces whereby the mass or fluidized bed possesses the characteristics of a liquid. These fluidized beds are conventionally produced by effecting a flow of a fluid such as a gas through a porous or perforated plate or membrane underlying the particle mass, at a sufficient rate to support the particles against the force of gravity. Conditions at the minimum fluid flow required to produce the fluid-like, or fluidized conditions, i.e., the incipient fluidization point are dependent on many parameters including particle size, particle density, etc. The increase in the fluid flow beyond the incipient fluidization point causes an expansion of the fluidized bed to accommodate the increased fluid flow until the fluid velocity exceeds the free falling velocity of the particles which are then carried out of the apparatus, a condition known as entrainment.
Recently, U.S. Pat. No. 4,115,927, described a process for stabilizing such fluidized beds against bubble formation by use of an axially applied magnetic field. The magnetically stabilized fluidized beds (MSFB) disclosed in the '927 patent are useful in carrying out the above-mentioned processes and particularly for removing solid particulates entrained in gaseous fluids. One problem connected with using the MSFB of the type disclosed in the '927 patent for particulate capture processes or other processes involving particulate containing gas streams, however, relates to plugging of the grid or perforated plate underlying the particle mass, thus causing substantial pressure drops to develop during operations as well as causing lost operating time while the grids are periodically cleaned.
A number of patents describe processes for the separation of particulates entrained in gaseous fluids by magnetic means. Such processes, quite often require the particulates themselves to be magnetic. One such process is described in U.S. Pat. No. 4,116,829. In this patent the gas containing the magnetic particulates entrained in the gas is passed through a chamber containing ferromagnetic filaments which are magnetized by an external magnetic field. The magnetic particulates adhere to the filaments. The particulates then are subsequently removed from the filaments.
Recently, J. R. Melcher at the Massachusetts Institute of Technology disclosed electrofluidized beds (EFB) for the collection of particulates entrained in a gaseous fluid. In such an EFB process, electrostatic fields are used to impress differential electric charges on the particulates to be captured so as to effect an attraction between such particulates and bed particles. Such processes are disclosed in U.S. Pat. Nos. 4,038,049 and 4,038,052 and the publications: Zahedi and Melcher, J. Air Pollut. Contr., (1974). While Melcher et al. refer to cross-flow and colinear EFB's by the term cross-flow EFB's, they actually describe contactors wherein the gas flow levitates the fluidizable particles as in the typical axial flow contactor. Typically, a cross-flow gas contactor is understood to be a contactor wherein the gas flows perpendicular to the external force field, i.e., gravity. The EFB's described by Melcher et al. have been shown to be useful collecting particulates, especially submicron particulates. However, the EFB's having a vertical gas flow are unable to process gas streams at velocities greater than 1 ft./sec. without encountering substantial pressure drops and/or entrainment of solids, e.g., using sand particles.
Cross-flow and panel bed fluid-solid contactors (i.e., where the fluid (gas) flow is perpendicular to gravity) are well-recognized means for contacting solids and fluids (particularly gases), the first industrial use being known as the Deacon process developed nearly 100 years ago. Perry's Chemical Engineers' Handbook, 5th Edition discloses details of several fluid cross-flow contactors of the type described by Dorfan, Squires and Zenz. Such processes eliminate the need for a porous grid such as is required in fluidized beds of Rosenweig and Melcher et al. As reported by Squires and Pfeffer (J. of the Air Pollution Control Association, Vol. 20, No. 8, pp. 534-538 (1970)) a considerable number of patents have been directed to cross-flow or panel bed devices. Squires et al. reported that many of these patents are directed to means for regulating the flow of the gravitating solid or means for withdrawing the solids. A number of patents have been granted on the use of panel beds as filters to remove particulates from gaseous streams. In these patents the panel beds are described as having each gas-entry surface free of loose surface particles of the filter solid. The surface is generally inclined at the solid particles' angle of repose, and it rests upon a louver. The solid particles used in these processes are generally rather large. U.S. Pat. No. 3,296,775 to Squires discloses that filter cake and a controlled amount of filter solid can be removed from each gas-entry surface by applying a surge backflow of gas from the clean side of the panel. Various improvements have been described in U.S. Pat. Nos. 3,410,055; 4,006,533; 4,004,896; 3,982,326; 4,004,350; 3,926,587; 3,926,593; 3,957,953, 3,981,355; 3,987,148; and 4,000,066.
U.S. Pat. Nos. 4,102,982 to Weir, Jr.; 4,017,278 and 4,126,435 to Reese also disclose processes for removing finely divided solids from gaseous streams by use of gas cross-flow contactors. The Reese patent discloses the use of louvered surfaces formed by perforating the walls to form louver vanes inclined to the vertical at angles ranging from 15.degree. to 80.degree..
U.S. Pat. No. 3,966,879 to Groenendaal et al. discloses a process for the removal of particulate matter and sulfur oxides from waste gases which comprises crosscurrent contacting of the waste gas stream with a moving bed of supported, copper-containing acceptor.
The panel-bed or cross-flow contactors described by Squires, Dorfan, Zenz, Groenendaal and others are limited by the amount of gas throughput than can be tolerated before solids break-through using small particles. Therefore, the degree in which the particles can come into contact with a gas is limited.
In some industrial processes, particularly flue gas desulfurization processes, relatively high superficial gas velocities, i.e., the order of 2-8 ft./sec. (60-245 cm./sec.) and higher with particles having a mean diameter size about 300-1000 microns are desired. The high velocities are desired so as to keep equipment of a practical size, despite the large volumes of gas to be processed and the small particle sizes facilitate intimate contacting of all of the gas with bed solids and effective use of all of the bed solids. Contactors capable of such high velocities with relatively small particles and at low pressure drops have hitherto not been described. There is, none the less, a genuine need for a fluid-solids contactor which will permit fluid-solids contacting of small particles at high superficial gas velocities without solids breakthrough or solids-entrainment. There is also a need for such a contactor having a relatively low pressure drop and which does not encounter the problems associated with grid plugging.